This invention relates generally to the field of conventional cylinder type door locks, and more particularly to an improved replacement housing of permutation type employing a plurality of combination tumblers adapted to substitute for an existing pin tumbler plug element within the substituted cylinder housing.
Pin tumbler constructions employ a key having notches along the shank portion thereof which correspond to the heights of cylinder plug pins in a given lock. When inserted into the plug through an exposed opening, the proper key aligns all of the parallel pin tumblers in the lock such that the parting planes of each tumbler are in concurrent relation with the surface of the plug permitting the plug to be turned to release a locking structure disposed there behind. If an incorrect key is inserted, a difference of as little as one sixty-fourth of an inch in any tumbler prevents the lock from turning. However, because of the presence of an opening in the exposed surface of the plug for the insertion of the key, the lock may be mechanically picked using a technique known as "torking" in which each tumbler is individually overcome in serial fashion by the insertion of an elongated picking tool.
Combination or mutation locks are well known in the lock art, and have been extensively used in padlocks and safe door constructions. While they are not totally immune to picking operations, considerably greater skills are necessary to overcome the combination tumblers which are not physically accessible to a potential lock-pick. At the present state of the art, very little has been done in the way of providing a substitute permutation type construction of configuration adapting it to be placed in cylindrical openings present in existing doors, whereby the likelihood of picking is reduced.
Reference is made to U.S. Pat. No. 4,064,718 granted Dec. 27, 1977 to Lloyd G. Cowen, one of the applicants herein, which discloses a related structure, the present application disclosing an improved construction.
In the above mentioned patent, provision is made for replacing the cylinder plug which is rotatably mounted within the lock housing. While this structure is not without utility, the relatively small diameter of the plug which the device replaces necessitates the making of the parts on a very small scale, and the combination tumblers are of correspondingly small diameter. As a result, relatively few numerical combinations are available, and, because of the presence of a relatively large number of gears and two separate parallel shafts, the cost of manufacture is relatively high.